Bacteriorhodopsin (bR) is a retinal protein that has been intensely studied over the years due to its inherent ability to function as a light-driven proton pump. It is composed of a 248 amino acid peptide with a molecular weight of 26,784 Da and it possesses a covalently attached retinal group. Structurally similar to the visual rhodopsin found in the human eye, bacteriorhodopsin is found in the cell membrane of Halobacterium salinarium where it functions to establish a proton and electrochemical gradient for the synthesis of ATP under anaerobic/light conditions. Hampp, Chem. Rev., 100 (5), 1755-76 (2000). Upon illumination with yellow-green light, a proton is pumped from the cytoplasmic side to the extracellular side of the cell membrane. Hwang, J. Membrane Bio., 33, 325-50, (1977). The charge separation resulting from the absorbed light appears 450 fs after the light incidence, implying the potential for a very high frequency sensor. Wang, Biosensors and Bioelectronics, 21, 1309-19, (2006).
For engineered applications, bacteriorhodopsin is purified as membrane patches, known as purple membrane (PM). Purple membrane is simply a large cell membrane patch, on average 500 nm in diameter, which is composed of multiple bacteriorhodopsin molecules and their associated lipids. Purple membrane is composed of bacteriorhodopsin and its associated lipids in a two-dimensional crystal. This structure provides it a high degree of chemical stability and resistance to thermal degradation. It is called PM due to its distinct purple color, which is due to its absorption peak near 570 nm.
Purple membrane has many unique properties that make it a viable engineering material. Specifically, PM has been shown to maintain functionality at temperatures up to 80° C. in water and 140° C. dried. Hampp, Chem. Rev., 100 (5), 1755-76 (2000). In the dried state, as well as the wet state, PM retains its light absorption properties and photochemical activity for years.
Quantum dots (QD) are semiconductor particles that have dimensions on the order of a few nanometers or less. When compared to other fluorophores, such as fluorescent dyes, QD's have higher quantum yields, greater chemical stability, longer lifetimes, and a greater resistance to photo bleaching. Jovin, Nature Biotechnology, 21, 32-33 (2003). They are also tunable based upon dot diameter. Another important characteristic is that QDs have a broad absorption range coupled with narrow emission spectra. The absorption spectrum for QDs lies primarily in the ultra-violet region while their emission range can be tuned to a very specific wavelength range. Medintz, Nature Materials, 2, 630-38, (2003). A QD's emission wavelength is directly dependent upon its diameter, with a diameter of approximately 2 nm yielding photonic emission in the 570 nm range.